Silver acetylide compound and process of making same



WITNESSES: f

0a. 4, 19.49. J, A, SHAW ET AL 2,483,440

SILVER ACETYLIDE COMPOUND AND PROCESS OF MAKING SAME Filed Feb. 7, 1946 E "III lNVEN EL TON FISHER,

' TORS JOSEPH A .SH

ATTORNEY Patented Oct. 4, 1949 ATLEN'T OFFICE SILVER .ACETYLIDE COMPOUND AND PROCESS OF MAKING SAME Joseph A. :ShawandElton Fisher, Pittsburgh, Pa., assignors to Koppers Company, Inc., Pittsburgh, Pa., a corporation 0f Delaware Application February 7, 1946, Serial No. 646,185

2 Claims. '1

This invention relates to a silveracetylide compound. More particularly the invention relates to a, non-explosive silver acetylide compound and the process of making the compound.

It is well known that silver nitrate is an elTective absorbent for olefines. However, the use of silver nitrate has been avoided commercially for absorption of olefines, such as ethylene and propylene from coke oven or coal gas because such gases also contain acetylene. Acetylene customarily reacts with the silver nitrate to form silver acetylide AgzCaAgNOs, which is a dangerous and powerful explosive. Arise in temperature to about 210 C. or an electrostatic spark are sumcient to detonate the dry substance.

Coke oven and coal gas contain hydrogen as Well as olefines and acetylene. If silver nitrate solution of strong concentration is used for absorbing the olefines, the hydrogen reduces the silver nitrate to deposit metallic silver from the absorbent solution with the consequent loss of expensive silver nitrate and interference with smooth mechanical operation of the ,process. For this reason dilute silver nitrate has been used as an absorbent solution, which was accompanied by the inevitableexplosion risk.

The present invention is based upon the discovery that a strong silver nitrate solution may be safely used for the absorption of olefines' that are associated with acetylene. Such strong silver to provide a new compound, silver acetylide Ag2C2.-6Ag-NO3, and the method of making the compound.

With this and other objects in view, the invention consists in the new silver acetylide compound AgzCafiAgNOa, and the process of making the acetylide. v

The principal application of the present inven- 2 tion is described in the co-pending application of Joseph A. Shaw, Ser. No. 637;240, filed December 1945, for Treatment of coke oven gas, now abandoned.

To produce the new compound silver acetylide, AgzCaSAgNOa, acetylene C2H2 is bubbled through solution of silver nitrate having a concentration of more than 20% anhydrous AgNOs in the solution. From many tests it has been found that a silver nitrate solution containing 31% anhydrous silver nitrate (AgNOs) gives a very desirable product. This strong silver nitrate solu tion will normally be reduced by hydrogen associated'with acetylene gas, particularly if coke oven or coal gas is being used as the source of acetylene. It has been found that if a small amount of ferric nitrate Fe(NO3)3, is contained in the silver nitrate solution, metallic silver will not be precipitated.

If acetylene is bubbled through a weak solution of silver nitrate, that is 10% or less by weight of anhydrous .AgNOz in the solution, a white crystalline compound AgzCaAgNOs is formed. This compound is very explosive under moderate heating and therefore involves risk in its formation and handling. The explosive silver acetylide is not very soluble in the dilute silver nitrate solution, but as the concentration of the silver nitrate solution is increased, the solubility of the explosive silver acetylide increases. As the strength of the silver acetylide solution used for absorption is increased up to 20% the explosive silver acetylide compound dissolves in the solution and tends to change its form from the explosive silver acetylide over to the safe silver acetylide compound AgzCaG'AgNOs.

In Table 1 is shown the solubility of the silver acetylide compound in the silver nitrate absorption solution and illustrates how the explosive silver acetylide is transformed into the safe silver acetylide compound.

TABLE- 1 Solubility of silver acetylide in its mother liquor Solubility Grams per Litre O u a of Crystals Type oi Mother Liquor (Aqueous Solutions) Trace AgZ'OZAgNO 0.1 N AgNOg With reference to Table 1 it may be stated that as the acetylene reacts with the silver nitrate solution, nitric acid is formed. Also nitric acid acts to minimize the precipitation of metallic silver by hydrogen. Commercially we are accordingly principally interested in acidified silver nitrate solutions. For this reason nitric acid was added to the solution in providing the data of Table 1. The presence of nitric acid in the solution tends to decrease the solubility of the silver acetylide compounds in the silver nitrate solution. However, the strength of the silver nitrate solution as it increases tends to compensate the reduction in solubility caused by the nitric acid. The mother liquor at the bottom of the table which consists of 31% silver nitrate solution, 3.2% of ferric nitrate solution, and 11% of nitric acid is the preferred adsorption solution which is used for absorbing acetylene and olefines from a gas containing hydrogen such as coke oven gas or coal gas. 1

It will be seen that the solubility of the silver acetylide in this absorption solution is less than in the or stronger silver nitrate solutions. With such an absorption solution, however, the silver nitrate will not be reduced to deposit metallic silver.

When th explosive silver acetylide compound is formed in very dilute silver nitrate solution, the substance, AG2C2AgNO3, appears as minute needle-like crystals as illustrated in Figure 1. If the silver nitrate concentration of the acetylene treated solution be about 3-5%, much larger and longer needles and crosses appear and if the strength of the solution be still further increased,

the small needles disappear leaving only the large A.

needles and crosses as illustrated in Figure 2. Careful chemical analysis has shown, however, that both of these classes of crystals have the chemical formula Ag'zCz-AgNOs and both are dangerous devastating explosives. tion of the silver solution be sufiiciently increased to or above, these crystals will go into solution. If the volume be sufficient, the solution will be permanent. Apparently the only way to obtain the AgzCzAgNOs complex from this solution is to redilute the solution or otherwise diminish the concentration of silver nitrate as by chemical means. If the volume is insufiicient to maintain a clear solution, a silver acetylide complex will be precipitated, but the precipitate will be the rhombohedral crystals of Ag2C26AgNO3, a sub stantially non-explosive material as illustrated in Figure 3.

Likewise, if acetylene or an inert gas containing acetylene be passed through a silver nitrate solution, a silver carbide precipitate will be formed. If the scrubbing solution has a low concentration of silver nitrate, only a small portion of the acetylene will be removed in any one scrubbing stage, as evidenced by the fact that many scrubbing stages may be placed in series and AgzCzAgNOa will appear in the last stage long before the silver nitrate in the first stage has been seriously depleted. But where solutions of about 25% silver nitrate or higher concentration are employed, no initial precipitation of silver carbide takes place at all. However, our tests show that even with coke oven gas carrying only 0.05% acetylene and passing at the rate 0.5 cubic feet per hour through a half inch diameter test tube containing about a one inch seal of silver nitrate, ninety-eight per cent of the acetylene is removed in the first pass. This is quite an unusual scrubbing eificiency and certainlyis evidence of a chemical combination of the acetylene with the If the concentra- Ill 4 silver nitrate, even though no precipitate is formed.

Based on this characteristic of strong silver nitrate solutions, we were able to design a rather excellent analytical method for the direct determination of even traces of acetylene in gas by direct scrubbing of a gas stream with strong silver nitrate solution, followed by high dilution with water, filtering and weighing of the AgzCzAgNOs. Previously no such method was available.

When acetylene addition to the previously mentioned strong solution is carried sufficiently far, a silver carbide does precipitate out in accordance with fixed solubility laws. This precipitate is wholl the rhombohedral crystal of Ag2C26AgNO3 shown in Figure 3 of the drawing. The

has a pronounced tendency to form supersaturated solutions in strong silver nitrate solution. A clear supersaturated solution may be maintained for several weeks, but upon vigorous shaking a copious precipitation of AgzCzGAgNOs will precipitate.

In Table 2 is shown the form of crystalline silver acetylides formed with different strengths of silver nitrate solutions. As might be expected in borderline concentrations two of these crystal forms can exist simultaneously but these areas are rather narrow.

TABLE 2 Form of precipitate expectancy [10 ml. AgNO; used at 25 C. in all instances] Vol. ml.

Results Determined Micro- CzHg Type of Solution scopicany) l5%dAgNO3 l N HNO3 o 20%dAgNOa l N HN O N o precipitate.

Needles with crosses.

N o precipitate.

A few rhombs.

Rhombs and a few crosses.

Mostly crosses but a few rhombs.

Rhombs only.

25% AgNOs l N HNOa.--

AgzCaBAgNOa is large rhombohedral crystals.

The silver acetylide AgzCaBAgNOi has been referred to as a safe acetylide compound. This compound will decompose when heated to a moderate temperature but the decomposition is not a violent explosive decomposition such as the explosion or decomposition of the compound AgaCaAgNOa when heated to its detonative temperature. The compound AgzczfiAgNos is in the form of comparatively large white rhomboheclral crystals which, in polarized light, exhibit a high degree of coloration, but are water-white in transmitted light. If the rhombohedral crystals or a strong silver nitrate solution containing the rhombohedral crystals is diluted with water, the rhombohedral crystals will be decomposed and form the explosive type of silver acetylide crystals.

The rhombohedral crystals may be separated from the solution in which they are precipitated by decantation or filtration. However, the silver nitrate solution always contains some silver acetylide in solution. In order to separate all of the silver acetylide from the silver nitrate solution, the acetylide compound may be precipitated by mercuric nitrate to form a mercury acetylide silver nitrate compound HgC2.3AgNO3, which may then be separated from the silver nitrate solution by decantation or filtration or the acetylene may be destroyed by boiling the above mercury precipitate with an excess of mercuric nitrate and nitric acid.

X-ray diffraction analyses were made of the Ag2Cz.AgNO3 and AgzCaGAgNOs. The lines on the stable salt Ag2C2.6AgNO3 difiraction analyses were quite clear and distinct and could be taken with a comparatively short exposure. On the other hand, the lines on the X-ray diffraction analyses of the explosive compound AgzCaAgNOs were very faint and, in fact, a very long exposure of the camera was required to get any picture. The heat energy of the X-ray camera apparently acted to decompose the salt while the exposure was being taken. Many attempts were made to get a good exposure but it would appear that the salt would decompose before an analysis showing the lines could be made.

The preferred form of the invention having been thus described, what is claimed as new is:

We claim:

1. A silver acetylide Ag2C2.6AgNO3 crystallizable as white rhombohedral crystals, which are readily soluble in strong silver nitrate solutions.

2. A process of making silver acetylide AgzCzfiAgNOa comprising: absorbing acetylene in an aqueous solution of silver nitrate containing nitric acid in which the concentration of silver nitrate during absorption is maintained at greater than 20% by weight of the solution.

JOSEPH A. SHAW. ELTON FISHER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,875,924 Horsley Sept. 6, 1932 2,150,349 Van Peski et a1. Mar. 14, 1939 OTHER REFERENCES Stettbacher I, Zentralblatt Chem. H, 1919, pages 126 and 127.

Stettbacher II, Chem. Abstracts, vol. (1941), page 3091.

Chavastelon, Compte rendu, vol. 124, pages 1364 and 1365 (1897).

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